[GHC] #12926: ghc master (8.1.20161202) panics with assertion failure with devel2 flavor and -O2

[GHC]

#12926: ghc master (8.1.20161202) panics with assertion failure with devel2 flavor
and -O2
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        Reporter:  pacak             |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 I can't reproduce this with GHC 8.1.20161207. Do I need to do something
 differently?

 {{{
 $ ghc/inplace/bin/ghc-stage2 -O2 Bug.hs -fforce-recomp
 [1 of 1] Compiling A                ( Bug.hs, Bug.o )

 Bug.hs:20:10: warning: [-Wmissing-methods]
     • No explicit implementation for
         ‘basicOverlaps’, ‘basicInitialize’, and ‘basicUnsafeRead’
     • In the instance declaration for
         ‘MVector Data.Vector.Unboxed.Base.MVector A’

 Bug.hs:28:10: warning: [-Wmissing-methods]
     • No explicit implementation for
         ‘Data.Vector.Generic.Base.basicUnsafeFreeze’,
         ‘Data.Vector.Generic.Base.basicUnsafeThaw’,
         ‘Data.Vector.Generic.Base.basicLength’,
         ‘Data.Vector.Generic.Base.basicUnsafeSlice’, and
 ‘Data.Vector.Generic.Base.basicUnsafeIndexM’
     • In the instance declaration for
         ‘Data.Vector.Generic.Base.Vector Data.Vector.Unboxed.Base.Vector
 A’
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12926#comment:4>
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